Semiconductor manufacturing technology has advanced to the point where a single die may contain millions of active devices. A key requirement for fabricating such high density circuits is the elimination of contaminants from the manufacturing process. This has led to the development of ultra high vacuum processing and closed manufacturing systems. Such closed systems preferably include insitu process sequences that can be precisely controlled without exposure of the wafer to the ambient.
One area of semiconductor manufacture in which the elimination of contaminants and the precise control of process sequences is important is in the etching techniques for etching different film layers formed on the wafer. In general, integrated circuits are formed by patterning regions on a semiconducting substrate and then by patterning different film layers formed on the substrate. As an example, during manufacture, a silicon substrate is typically formed with an oxide layer, such as silicon dioxide. This oxide layer may function as a gate oxide to the active devices formed on the substrate. In addition this oxide layer may function as the dielectric layer for capacitors formed on the substrate.
Other film layers may be formed or deposited on the gate oxide. As an example, polysilicon may be deposited on the gate oxide layer as a surface conduction layer. Other films in turn, may be deposited on the polysilicon layer. These various film layers must be patterned and etched to the gate oxide.
The technique of photolithography is frequently used to pattern and etch these different film layers. Typically this involves coating the wafer with a photoresist. The photoresist is then exposed with ultraviolet radiation passed through a mask. A desired pattern is thus imaged on the photoresist to form a photoresist mask. The photoresist mask includes exposed areas that allow an underlying film to be etched using wet or dry etching processes. The etch depth or endpoint must be closely controlled to insure that an underlying layer (i.e. gate oxide) is not also etched through. For etching the small feature sizes required for high density applications, dry etch processes are typically utilized. With dry etching, gases are the primary etch medium. Plasma dry etching uses plasma energy to drive the reaction.
As the industry moves towards higher density applications, the gate oxides used for the active devices of a semiconductor structure have tended to become thinner. Such thin gate oxides require etching techniques and etchants that are highly selective to the gate oxide. There is then a need in the industry for better methods for patterning and etching the layers of a semiconductor structure, particularly polysilicon which have been formed on a thin gate oxide.
It is known in the industry that in a plasma dry etch process, the etch selectivity to a gate oxide can be more easily achieved when there is no photoresist present during the polysilicon to gate oxide etch step. Accordingly, in a representative prior art process sequence for etching a semiconductor structure that includes a gate oxide, another oxide layer is first deposited over the semiconductor structure. A layer of photoresist in then deposited on the oxide layer. An oxide hard mask is formed to the polysilicon layer by etching the oxide layer through the photoresist mask. For stripping the photoresist, the wafer is transferred to a photoresist strip chamber. With the photoresist removed, the wafer is transferred to a poly etch chamber to etch the polysilicon layer to the gate oxide.
The transfer of the wafer during the different etch steps tends to introduce contaminants during this process. In particular, exposure of the wafers to ambient may introduce contaminants. Additionally, each different process chamber may introduce contaminants. Moreover, process parameters are difficult to control with physical transfer of the wafers between these different process stations. Finally, the operation of these different process stations is time consuming and adds to production costs.
The present invention recognizes that during a plasma dry etch, a photoresist mask may be stripped insitu by using an ozone plasma. The etch selectivity of a polysilicon layer with respect to a gate oxide can thus be increased without exposing the wafer to multiple process stations.
It is known in the art that ozone is effective for removing a layer of photoresist from an underlying surface. As an example, U.S. Pat. Nos. 4,341,592; 4,885,047; and 5,071,485 disclose semiconductor manufacturing processes in which ozone is used to strip photoresist from a silicon substrate. The method of the invention uses this technique to provide an improved method for etching through a polysilicon layer to a gate oxide.
Accordingly, it is an object of the present invention to provide a method for stripping a photoresist layer and etching through a polysilicon layer to a gate oxide. It is a further object of the present invention to provide a method for etching through a polysilicon layer to a gate oxide that can be performed insitu so that contamination can be eliminated. It is another object of the present invention to provide a method of plasma dry etching a polysilicon layer to a gate oxide which is relatively inexpensive and adaptable to large scale semiconductor manufacture.